On July 7, Hasib Hussain, Shahzad Tanweer, and Mohammad Sidique Khan traveled to Luton, north of London, to meet with Jermaine Lindsay.

According to experts who have been briefed by police, they parked a rental car outside the train station and paid to park for seven days, then purchased roundtrip tickets to London. Police later found explosives in the car.

When the men arrived at King's Cross station in the British capital, they split up and attacked three Tube trains and a bus -- killing 52 people and themselves.

No suicide notes or videotaped messages for their families have surfaced as has happened in attacks elsewhere.

"The roundtrip tickets, the fact that one of them spent a lot recently repairing his car and one of them had a family and was the teacher of the disabled and underprivileged children, it doesn't ring right," said Paul Beaver, a security and defense expert in London with close police contacts. "If you had that much commitment, how are you going to take your life? It's happened in Palestine, but these people were brought up in the UK."

The four men could have been duped into thinking they were carrying timed bombs and would be able to get away, Beaver said. He pointed to the fact that the bombs would have caused much more damage if detonated in stations, which could suggest they were not expected to immediately explode.

"The thing is we won't know that until we knew more about the detonation systems, which is what the forensic boys are doing now," he said.

Beaver said he did not put much stock in the theory that three of the bombs must have had timers because they exploded in quick succession. "It could be, but it isn't very difficult to synchronize your watches," he said.

The bombers' behavior also could have been designed to avoid suspicion, Beaver said. "The more normal you can be, the less conspicuous you are -- you have to be a gray man," he said.

The same tactic was used by the Sept. 11 suicide pilots who lived and studied in Hamburg, Germany -- Mohamed Atta, Marwan al-Shehhi and Ziad Jarrah -- who used legal passports, dressed as Westerners and did nothing to draw the attention of authorities.

The Sept. 11 hijackers bought one-way plane tickets. Since the attacks on the United States, airlines have been running security checks on passengers who purchase one-way tickets or pay with cash.

The July 7 bombers could simply have been cautions, said Alex Standish, editor of Jane's Intelligence Digest. Also, the least expensive Luton to London ticket costs only about 15 cents more for a return fare.

Palestinian suicide bombers often leave videotaped messages. But the July 7 attackers grew up in Britain and would have known their families could be charged as accessories if told of the planned attacks, Standish said.

The fact that they carried identification -- cited by some as evidence they planned to flee -- was more likely designed to ensure the world would know who they were, he said.

"This is not something which is done secretly," Standish said of suicide bombings.

Although London police have not yet said whether the July 7 attacks were linked to the failed attacks July 21, police chief Ian Blair did say there was a "resonance" between the two.

In the latter attempts, it's clear at least some of the bombers expected to die -- they lay on top of their explosives, which failed to go off.

Abisha Moyo saw one attempted attack on a subway train. He told the Daily Mail newspaper he broke off a cell phone call when he heard a noise like a pistol shot.

"I turned around and there was a man lying on the ground with his arms outstretched in a Jesus Christ position, lying on top of a medium-sized black and green rucksack, face up," he said. "I thought he might have been shot. I went up to him and said: 'Are you all right mate?' But he just ignored me."

London police would not comment into whether they were looking into the possibility that the July 7 attackers had not intended to die.

But Standish said none of his police contacts have indicated that they think July 7 is anything but a suicide attack.

"These guys were mass murderers," he said. "They intended to carry out what they did. If they were innocent dupes or fooled? I think that's a cockeyed theory."

However, Beaver said there are too many questions to know for sure.

"The answer is right now this is really all speculation, and we really won't know until the forensic evidence is in," he said.